Don’t set combined API on Notify python client

Because we’re setting the API key and service ID after calling the
`__init__` method of the client it wasn’t doing the thing where it
splits the combined key into the two individual UUIDs. So we still need
to set them directly, individually on the client.

app/notify_client/invite_api_client.py

@@ -9,10 +9,8 @@ class InviteApiClient(NotifyAdminAPIClient):
 
     def init_app(self, app):
         self.base_url = app.config['API_HOST_NAME']
-        self.api_key = '{}-{}'.format(
-            app.config['ADMIN_CLIENT_USER_NAME'],
-            app.config['ADMIN_CLIENT_SECRET'],
-        )
+        self.service_id = app.config['ADMIN_CLIENT_USER_NAME']
+        self.api_key = app.config['ADMIN_CLIENT_SECRET']
 
     def create_invite(self, invite_from_id, service_id, email_address, permissions):
         data = {